This one is absolutely baffling to me, but apparently we plebes are not to be trusted with the power inherent in a flash briefing card's subtitle.
If you compare the card output from a TuneIn flash briefing skill with that of a standard third party flash briefing skill, you'll see that there's some extra subtext on the TuneIn card (there are actually two things - subtext on the title and on the image). We spent several hours trying to reverse engineer this and see if the "subtitle" field was undocumented, without luck - as far as we can tell it's just not available.
Seriously - it's a text field. We already have a free form text field for the main body of content, so there's no way people can abuse this for ill. Instead, we just don't have the tools to make our content look less ugly.
Why are we being given a subpar implementation of this feature?